Understanding Dissimilar Metal Corrosion
There’s an often-overlooked threat to the buildings we occupy every day: metal corrosion. Every year, corrosion costs the world $2.5 trillion due to damage of steel buildings and infrastructure. It’s important to understand the causes and prevention of corrosion and how with proper planning, you can save future headache.
What is Galvanic Corrosion?
Galvanic corrosion, also known as dissimilar metal corrosion or bimetallic corrosion, is an electrochemical process that leads to corrosion. It comes into play when two different metals are immersed in a conductive solution or in direct contact with each other. When these metals are in electrical contact and in the presence of an electrolyte, such as moisture, one of the metals experiences accelerated corrosion while the other remains protected.
All metals have different electrical potentials. The galvanic series determines the nobility, or corrosion resistance, of various metals. There’s greater potential for galvanic reaction, or corrosion, when metals are farther apart on the scale. Take stainless steel and zinc, which are on opposite ends of the galvanic series. If these two different metals come in direct contact, the more noble metal—stainless steel—resists corrosion. Meanwhile, the less noble metal—zinc—corrodes faster than it otherwise would if isolated.
If this reactivity is ignored in a structure, uncontrolled corrosion becomes a serious problem. Metal components weaken, leading to the compromise or failure of building elements, and in some cases, excessive moisture intrusion into the building damages critical structural members.
Risks of Dissimilar Metal Corrosion in Buildings
The risks associated with dissimilar metal corrosion in buildings are significant. It's not just the structural integrity of the buildings that's compromised. The safety of inhabitants is also at stake.
The top three risks of dissimilar metal corrosion include:
Structural Weakness
When metal components suffer corrosion, they weaken and can potentially collapse the entire structure. This not only poses risks to the building's stability but can also create safety issues.
Higher Maintenance Costs
The need for frequent replacement or repair of corroded sections can skyrocket maintenance expenses, affecting the overall economic viability of the building.
Aesthetic Damage
Galvanic corrosion can affect exterior metal architectural elements. This can lead to unsightly staining and discoloration, impacting the appearance and value of the property.
Recognizing Common Forms of Corrosion
There are many types of metal corrosion. The deterioration varies depending on the physical and chemical properties of the metals involved and their exposure to environmental conditions. Some common examples include:
Rusting Iron and Steel Structures: Rusting is an unmistakable sign of corrosion. It particularly affects iron or steel structures and can lead to an array of structural issues, ranging from minor aesthetic concerns to major structural failures. This can affect areas such as roofing, staircases, or balconies, where exposure to moisture is especially high.
Corrosion of Copper Pipes: Copper pipes, despite the nobility and durability of the metal, are not immune to corrosion. When these come into contact with dissimilar metals like steel or aluminum, the corrosion potential escalates. This often results in leaking pipes, water damage, and expensive repair work.
Deterioration of Zinc Layers: Zinc coatings are frequently employed on steel structures to fortify them against the effects of corrosion. However, even these protective layers can deteriorate over time, exposing the underlying steel to corrosion.
The Role of Metals in Roofing
Roofs are perhaps the most exposed and vulnerable aspect of a building. You must consider how different metals interact electrochemically with each other in order to prevent dissimilar metal corrosion and long-term roofing damage.
The choice of metals for construction involves consideration of durability, resilience, and resistance to environmental elements. Some common choices are:
Aluminum: Aluminum is a popular roofing material because it’s lightweight and highly corrosion resistant. But when aluminum is in contact with other metals, especially in the presence of moisture such as saltwater, it can become susceptible to galvanic corrosion.
Steel: Although steel is prone to rust, coating or galvanizing the metal makes it corrosion resistant. One example of this is a hot-dip galvanized coating primarily comprised of zinc and zinc alloys. But the likelihood of galvanic corrosion increases when these protective layers wear off or get damaged.
Copper: Copper has remarkable longevity and resistance to corrosion. When maintained, copper roofs can last for centuries. However, copper roofs become vulnerable to galvanic corrosion when they come in contact with different metallic elements.
Lead: Lead is known for its longevity and flexibility, making it one of the oldest flashing materials in roofing. But lead is susceptible to corrosion when it comes into contact with other metals.
How to Prevent Dissimilar Metal Corrosion
Understanding the factors that catalyze dissimilar metal corrosion can guide your prevention efforts. Variations in temperature, exposure to chemicals, and even air pollutants can accelerate the corrosion rate.
You can help prevent galvanic corrosion by:
Avoiding Direct Contact: Refrain from direct metallic contact. You can use non-metallic materials such as washers or gaskets to create a barrier between dissimilar metals. This step interrupts the electrical pathway, thereby reducing the chances of corrosion.
Using Protective Coatings: Applying protective coatings or paints can shield metal surfaces from direct exposure to moisture. This is an affordable yet effective line of defense against corrosion. The options range from special anti-corrosion paints to more advanced powder coatings, each offering different degrees of protection.
Employing Cathodic Protection: Cathodic protection works by placing external devices to create a circuit and provide a source of electrons to a steel structure. This strategy takes advantage of the very nature of galvanic corrosion. A “sacrificial” metal like magnesium or zinc willingly corrodes to safeguard the more crucial component. Corrosion moves to this anode metal and extends the life of the primary metal.
Overseeing Construction: Architects may specify certain materials in designs, such as stainless steel screws, to prevent corrosion when connecting dissimilar metals. If construction crews miss these specifications, it can lead to problems down the road, especially for critical components like roofing. Diligent oversight and inspection by the design team during the construction administration phase is crucial.
Defense against dissimilar metal corrosion is a blend of foresight, design, and maintenance. In order to design for longevity, you must consider the potential for galvanic corrosion at the start of a project. This includes responsibly selecting materials and accounting for potential contact points between metals. Proper corrosion management can enhance your building’s lifespan and structural integrity.